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This is a review of high-frequency methods for measuring and controlling the humidity of various materials. The moisture of cotton seeds is a major factor affecting their qualitative and quantitative characteristics in technologies for their storage, transport, and processing. Thus, direct humidity measurements of cotton seeds during processing is an important problem. A mathematical model of a high-frequency hygrometer for cotton seeds is constructed in which the test material is represented as a complicated dielectric in an electric field. A substitution circuit containing a capacitance between electrodes is proposed, along with capacitances and resistances corresponding to different forms of polarization. Four variants of the substitution circuit are examined which approximately describe the dependence of the tangent of the dielectric loss angle on the frequency of the electromagnetic field. It is shown that the simplest parallel two-component RC substitution circuit corresponds most closely to the actual object of measurement at frequencies of 105–108 Hz. [ABSTRACT FROM AUTHOR]

A joint project between the National Institute of Metrology Thailand (NIMT) and the Thai Meteorology Department (TMD) was established for improving the traceability of meteorology measurements at automatic weather stations (AWSs) in Thailand. The project aimed to improve traceability of air temperature, relative humidity and atmospheric pressure by implementing on-site calibration facilities and developing of new calibration procedures. First, new portable calibration facilities for air temperature, humidity and pressure were set up as working standard of the TMD. A portable humidity calibrator was applied as a uniform and stable source for calibration of thermo-hygrometers. A dew-point hygrometer was employed as reference hygrometer and a platinum resistance thermometer (PRT) traceable to NIMT was used as reference thermometer. The uniformity and stability in both temperature and relative humidity were characterized at NIMT. A transportable pressure calibrator was used for calibration of air pressure sensor. The estimate overall uncertainty of the calibration setup is 0.2 K for air temperature, 1.0 % for relative humidity and 0.2 hPa for atmospheric pressure, respectively. Second, on-site calibration procedures were developed and four AWSs in the central part and the northern of Thailand were chosen as pilot stations for on-site calibration using the new calibration setups and developed calibration procedures. At each station, the calibration was done at the minimum temperature, average temperature and maximum temperature of the year, for air temperature, 20 %, 55 % and 90 % for relative humidity at the average air temperature of that station and at a one-year statistics pressure range for atmospheric pressure at ambient temperature. Additional in-field uncertainty contributions such as the temperature dependence on relative humidity measurement were evaluated and included in the overall uncertainty budget. Preliminary calibration results showed that using a separate PRT probe at these AWSs would be recommended for improving the accuracy of air temperature measurement. In case of relative humidity measurement, the data logger software is needed to be upgraded for achieving higher accuracy of less than 3 %. For atmospheric pressure measurement, a higher accuracy barometer traceable to NIMT could be used to reduce the calibration uncertainty to below 0.2 hPa. [ABSTRACT FROM AUTHOR]

A method is proposed for calibrating microwave hygrometers with lower error in determining the difference between the true conversion function and the nominal calibration dependence attributed to a series of hygrometers. The proposed method can be used to extend the results of calibrating a single hygrometer with a natural sample of material to an entire series of hygrometers. [ABSTRACT FROM AUTHOR]

The Russian national humidity standard of gases has been modernized in order to increase the number of reproducible quantities of humidity (relative humidity, dew/frost-point temperature, mole fraction) and to extend the humidity and operating temperature ranges. The basis of the standard comprises two humidity generators with operating temperature ranges from $$5\,^{\circ }\hbox {C}$$ to $$90\,^{\circ }\hbox {C}$$ and from $$-60\,^{\circ }\hbox {C}$$ to $$15\,^{\circ }\hbox {C}$$ . The common working range (from $$5\,^{\circ }\hbox {C}$$ to $$15\,^{\circ }\hbox {C}$$ ) allows comparison of the generators. The generators use the two-pressure method to generate humid gas defined in terms of the relative humidity (from 5 %rh to 98 %rh at temperatures from $$90\,^{\circ }\hbox {C}$$ to $$-60\,^{\circ }\hbox {C}$$ ) and the one-pressure (or phase equilibrium) method to generate humid gas defined in terms of the vapor mole fraction (from 0.6 ppm to $$700\times 10^{3}$$ ppm) and dew/frost-point temperature (from $$-79\,^{\circ }\hbox {C}$$ to $$90\,^{\circ }\hbox {C}$$ ). The expanded uncertainty in the relative humidity is no more than 0.2 %rh, no more than 1.2 % in the vapor mole fraction, and no more than $$0.12\,^{\circ }\hbox {C}$$ in the dew/frost-point temperature. The ordinary hygrometers are traceable to the national primary standard in accordance with the state hierarchical chain for measuring means of gas humidity. The state hierarchical chain consists of three branches for means of measurements: (a) mole fraction, (b) dew/frost-point temperature, and (c) relative humidity with each branch represented as the scheme: primary standard-secondary standard-working standard-ordinary hygrometer. Calibration and verification of working standards and ordinary hygrometers, and their traceability to the primary standard use methods of (i) direct measurements, (ii) direct comparison, or (iii) comparison with a comparator. [ABSTRACT FROM AUTHOR]

A collimated surface acoustic wave (SAW) circles around the equator of a sphere hundreds of times. Because of the long distance travel of the collimated SAW, a small change in the SAW propagation caused by the environment of the sphere can be accumulated as a measurable range in amplitude and/or in delay time. So, a spherical SAW device enables highly sensitive water-vapor measurements. In this paper, deep sub $$\upmu \hbox {mol}{\cdot }\hbox {mol}^{-1}$$ water-vapor detection by 1 mm diameter quartz crystal ball SAW sensors is described. To measure such a low water-vapor concentration in real time, it is necessary to compensate the temperature dependence of the ball SAW sensor, which is about 20 $$\hbox {ppm}{\cdot }^{\circ }\hbox {C}^{-1}$$ in delay time change. A dual-frequency burst analog detector was developed for the temperature compensation in real time. By using a harmonic SAW sensor, which was excited by 80 MHz and 240 MHz at the same time, it was confirmed that the delay time drift for a temperature range of $$21.0\, ^{\circ }\hbox {C} \pm 1.0\,^ {\circ }\hbox {C}$$ became less than 0.05 ppm in delay time change. By using dual-ball SAW sensors (which included a 150 MHz sensor with a water-vapor sensitive layer and a 240 MHz sensor as a reference), water-vapor concentrations from 0.1 $$\upmu \hbox {mol}{\cdot }\hbox {mol}^{-1}$$ to $$5\; \upmu \hbox {mol}{\cdot }\hbox {mol}^{-1}$$ were successfully measured. It appears that the delay time change is proportional to the square root of the water-vapor concentration. The detection limit determined by the electrical noise of the system was estimated at $$0.01\; \upmu \hbox {mol}{\cdot }\hbox {mol}^{-1}$$ . [ABSTRACT FROM AUTHOR]

The Centre for Metrology and Accreditation (MIKES) is developing a temperature-humidity calibration system for radiosondes. The target minimum air temperature and dew-point temperature are $$-80\,^\circ $$ C and $$-90\,^\circ $$ C, respectively. When operating in this range, a major limiting factor is the time of stabilization which is mainly affected by the design of the measurement chamber. To find an optimal geometry for the chamber, we developed a numerical simulation method taking into account heat and mass transfer in the chamber. This paper describes the method and its experimental validation using two stainless steel chambers with different geometries. The numerical simulation was carried out using Comsol Multiphysics simulation software. Equilibrium states of dry air flow at $$-70\,^\circ $$ C with different inlet air flow rates were used to determine the geometry of the chamber. It was revealed that the flow is very unstable despite having relatively small Reynolds number values. Humidity saturation abilities of the new chamber were studied by simulating water vapor diffusion in the chamber in time-dependent mode. The differences in time of humidity stabilization after a step change were determined for both the new chamber model and the MIKES Relative Humidity Generator III (MRHG) model. These simulations were used as a validation of the simulation method along with experimental measurements using a spectroscopic hygrometer. Humidity saturation stabilization simulations proved the new chamber to be the faster of the two, which was confirmed by experimental measurements. [ABSTRACT FROM AUTHOR]

The goal of interlaboratory comparisons is to verify the competence of accredited or non-accredited laboratories, including verification of the reported measurement uncertainties, whenever possible. An interlaboratory comparison consists of measurement results of a laboratory and a reference laboratory which is competent to run the comparison and provides traceability at the required uncertainty level. In Turkey, the demand for an interlaboratory comparison in the humidity field increased drastically in parallel to an increasing number of accredited laboratories in this field. Therefore, a national interlaboratory comparison in the scope of relative humidity in the range between 11 %rh and 95 %rh at $$23\, {}^{\circ }\hbox {C}$$ was initiated by TUBITAK UME in 2011. Two hygrometers were calibrated at TUBITAK UME in the above-mentioned range and characterized in terms of long-term stability, temperature dependence, and hysteresis. One of the hygrometers is used throughout this comparison. The second one was kept aside as a backup device in case of a malfunction of the first device. Organization and evaluation of the interlaboratory comparison in the scope of relative humidity were performed according to the EN ISO/IEC 17043:2010 'Conformity assessment-general requirements for proficiency testing' among thirteen national laboratories. Evaluation of comparison results are performed by normalized error analysis. It was found that among the 95 realized points of comparison, 85 (89 %) had $$E_{n} \le 1$$ and only 10 (11 %) had $$E >1$$ . [ABSTRACT FROM AUTHOR]

We present validation studies of MLS V2.2 and V3.3 water vapor (WV) and ozone profiles over the Tibetan Plateau (Naqu and Lhasa) and its adjacent region (Tengchong) respectively by using the balloon-borne Cryogenic Frost point Hygrometer and Electrochemical Concentration Cell ozonesonde. Coincident in situ measurements were selected to compare the MLS V2.2 and V3.3 WV and ozone profiles for understanding the applicability of the two version MLS products over the region. MLS V2.2 and V3.3 WV profiles respectively show their differences within −2.2±15.7% ( n=74) and 0.3±14.9% ( n=75) in the stratosphere at and above 82.5 hPa. Accordingly, at 100 hPa, the altitude approaching the tropopuase height, differences are within 9.8±46.0% ( n=18) and 23.0±45.8% ( n=17), and they are within 21.5±90.6% ( n=104) and 6.0±83.4% ( n=99) in upper troposphere. The differences of MLS ozone are within −11.7±16.3% ( n=135, V2.2) and 15.6±24.2% ( n=305, V3.3) at and above 82.5 hPa. At 100 hPa, they are within −3.5±54.4% ( n=27) and −8.7±41.6% ( n=38), and within 18.0±79.1% ( n=47) and 34.2±76.6% ( n=160) in the upper troposphere. The relative difference of MLS WV and ozone profile has significant oscillation and scatter at upper troposphere and lower stratosphere partly due to the stronger gradients of WV and ozone concentrations here as well the linear interpolation of sonde data for the intercomparison. At and below 70 hPa, the relative differences of MLS ozone are significantly larger over Lhasa during the Tibetan Plateau 'ozone valley' season, which is also the Asian Summer Monsoon period. The MLS ozone differences over the three sites are similar in their vertical distributions during that period. A simple linear correlation analysis between MLS and sonde profiles indicates that the sensitivity of MLS profile products is related to concentrations at each pressure level. The MLS V3.3 product sensitivity is slightly improved for WV at and above 82.5 hPa, whereas it is not obvious for ozone. The possible factors contributing to the differences of the MLS profile products of WV and ozone are discussed. [ABSTRACT FROM AUTHOR]

An advanced in situ diode laser hygrometer for simultaneous, sampling-free detection of interstitial HO and HO vapor was developed and tested in the aerosol interaction and dynamics in atmosphere (AIDA) cloud chamber during dynamic cloud formation processes. The spectrometer to measure isotope-resolved water vapor concentrations comprises two rapidly time-multiplexed DFB lasers near 1.4 and 2.7 µm and an open-path White cell with 227-m absorption path length and 4-m mirror separation. A dynamic water concentration range from 2.6 ppb to 87 ppm for HO and 87 ppt to 3.6 ppm for HO could be achieved and was used to enable a fast and direct detection of dynamic isotope ratio changes during ice cloud formation in the AIDA chamber at temperatures between 190 and 230 K. Relative changes in the HO/HO isotope ratio of 1 % could be detected and resolved with a signal-to-noise ratio of 7. This converts to an isotope ratio resolution limit of 0.15 % at 1-s time resolution. [ABSTRACT FROM AUTHOR]

A new spatially scanning TDLAS in situ hygrometer based on a 2.7-µm DFB diode laser was constructed and used to analyse the water vapour concentration boundary layer structure at the surface of a single plant leaf. Using an absorption length of only 5.4 cm, the TDLAS hygrometer permits a HO vapour concentration resolution of 31 ppmv. This corresponds to a normalized precision of 1.7 ppm m. In order to preserve and control the HO boundary layer on an individual leaf and to study the boundary layer dependence on the wind speed to which the leaf might be exposed in nature, we also constructed a new, application specific, small-scale, wind tunnel for individual plant leaves. The rectangular, closed-loop tunnel has overall dimensions of 1.2 × 0.6 m and a measurement chamber dimension of 40 × 54 mm (H × W). It allows to generate a laminar flow with a precisely controlled wind speed at the plant leaf surface. Combining honeycombs and a miniaturized compression orifice, we could generate and control stable wind speeds from 0.1 to 0.9 m/s, and a highly laminar and homogeneous flow with an excellent relative spatial homogeneity of 0.969 ± 0.03. Combining the spectrometer and the wind tunnel, we analysed (for the first time) non-invasively the wind speed-dependent vertical structure of the HO vapour distribution within the boundary layer of a single plant leaf. Using our time-lag-free data acquisition procedure for phase locked signal averaging, we achieved a temporal resolution of 0.2 s for an individual spatial point, while a complete vertical spatial scan at a spatial resolution of 0.18 mm took 77 s. The boundary layer thickness was found to decrease from 6.7 to 3.6 mm at increasing wind speeds of 0.1-0.9 m/s. According to our knowledge, this is the first experimental quantification of wind speed-dependent HO vapour boundary layer concentration profiles of single plant leaves. [ABSTRACT FROM AUTHOR]

An improved bubble column evaporator (BCE) system was used to determine accurate and precise enthalpy of vaporization (Δ H) values for concentrated salt solutions. The method is based on the steady state volumetric energy balance developed in a BCE. The BCE system offers a novel and simple approach for Δ H measurements because it only requires measurement of the hydrostatic differential pressure across the column and the temperatures of the steady state column and the inlet gas. In this work an automatic acquisition system was developed in order to study the frequency distribution of Δ H values. It was established that vacuum insulation of the bubble column and a data acquisition system enhanced both the accuracy and precision of Δ H measurements. In addition, a hygrometer was used to measure vapor pressures of concentrated LiSO and ZnSO solutions to determine new Δ H values. The results obtained are in better agreement with expected values and indicate the conditions required to enhance evaporative cooling systems and improve thermal desalination. An analysis was also carried out on the effect of salt concentration on Δ H and on calculated values of the partial molar enthalpy of dilution (Δ H). [ABSTRACT FROM AUTHOR]

Direct tunable diode laser absorption spectroscopy (dTDLAS) is a powerful diagnostic technique for absolute and accurate gas analysis with highest chemical specificity. Due to its first principles approach, dTDLAS is often claimed to be 'calibration-free', but this and the absolute accuracy has not been rigorously validated with respect to a high-accuracy reference. This work describes the first rigorous, side-by-side comparison of a dTDLAS hygrometer-called SEALDH-with a highly accurate, internationally validated, primary reference humidity generator (PHG), which also serves as the German national HO-standard. This PHG provides a humidified air stream with dew points between −30 °C and +60 °C with an uncertainty of 0.035 K (2 σ) (equivalent relative HO mixing ratio uncertainty: 0.4 %). Without any previous calibration, SEALDH was found to accurately reproduce the PHG reference values over the full range from 600 to 20,000 ppmv investigated in the 1-week lab study. Over this range, the SEALDH-PHG relative deviation was in average −1.45 %, the worst case being −2.5 % at 1,000 ppmv, the best −0.2 % at 600 ppmv. As SEALDH's relative uncertainty was metrologically determined to be 4.3 % ( k = 2), these deviations are for all concentration steps in full compliance with the PHG reference. Systematic contributions to the relative deviation could be correlated with line shape deviations between the measured line profile and the fitted Voigt line shape. Using this information, SEALDHs absolute accuracy can be improved further to down to an average relative deviation to the PHG of +0.21 %. [ABSTRACT FROM AUTHOR]

A new design of a sensitive and stable hygrometer sensor that implements a method of measurement of moisture content and relative air humidity based on the output voltage of an electrical circuit as a function of the current of a coronal discharge across the sensor's electrodes is proposed. [ABSTRACT FROM AUTHOR]

In this paper, the validation of the water dew-point generator with methane as a carrier gas in the temperature range from $$-41\,^{\circ }\hbox {C}$$ to $$+15\,^{\circ }\hbox {C}$$ and at pressures up to 6 MPa is reported. During the validation, the generator was used with both nitrogen and methane to investigate the effect of methane on the generator and the chilled mirror dew-point meters. The effect of changing the flow rate and the dew-point temperature of the gas entering the generator, on the gas exiting the generator was investigated. As expected, methane at high pressures created hydrates in combination with water and low temperatures, thus limiting the temperature range of the generator to $$+8\,^{\circ }\hbox {C}$$ to $$+15\,^{\circ }\hbox {C}$$ at its maximum operating pressure of 6 MPa. A lower operating pressure extended the temperature range; for example, at 3 MPa, the temperature range was already extended down to $$-15\,^{\circ }\hbox {C}$$ , and at 1 MPa, the range was extended down to $$-41\,^{\circ }\hbox {C}$$ . The validation showed that, in its operating range, the generator can achieve with methane the same standard uncertainty of $$0.02\,^{\circ }\hbox {C}$$ frost/dew point already demonstrated for nitrogen and air carrier gases. [ABSTRACT FROM AUTHOR]

We present here in situ measurements obtained between 1991 and 2011 in outer-vortex conditions by the ELHYSA balloon-borne frost-point hygrometer. The frost-point hygrometer profiles are used for comparisons with the satellite data from version 19 (v19) and version 3.3 (v3.3) of the HALogen Occultation Experiment (HALOE) and the Microwave Limb Sounder (MLS) respectively. Potential Vorticity mapping is applied to all data sets to remove contributions of transient tropical intrusions and polar vortex air masses and hence ensure consistent comparisons between the balloon and satellite observations. Our selected balloon in situ observations are too sparse to directly infer mid-latitude stratospheric time series for continuous comparisons with HALOE and MLS records or derive water vapour trends but can be used to validate the satellite data. A mean difference of −0.83 ± 1.58 % (−0.04 ± 0.07 ppmv) is obtained between HALOE v19 data and the balloon frost-point observations (with respect to HALOE) over the 30-80 hPa altitude range. The hygrometer-HALOE differences appear time-dependent as already presented in the literature. The mean difference reaches 2.80 ± 0.96 % (0.13 ± 0.04 ppmv) for MLS v3.3, with MLS systematically wetter than the balloon data reflecting a systematic bias between both datasets. We use our balloon data as reference to provide some information about the HALOE-MLS difference. From post-2000 ELHYSA-HALOE and ELHYSA-MLS comparisons, we find a HALOE-MLS difference matching the expected bias, with MLS v3.3 6.60 ± 2.80 % (0.27 ± 0.11 ppmv) wetter than HALOE v19. From the results obtained from our balloon-satellite data comparisons, we finally discuss the issue about merging the HALOE and MLS data sets to provide stratospheric water vapour trends. [ABSTRACT FROM AUTHOR]

The investigation of the hydromechanical behavior of unsaturated expansive soil depends on the determination of hysteretic soil water characteristic curves (HSWCCs). This research investigates the HSWCCs of two natural expansive soils initially compacted at optimum moisture content (OMC) and maximum dry density (MDD) and then saturated to their maximum expansion state to ensure identical capillary history. Filter paper, pressure plate and chilled mirror hygrometer suction measurement techniques were utilized, and corresponding measurements of three-dimensional volume change made. The cyclic swell-shrink paths were deduced in terms of volumetric strain versus either water content or suction. The cyclic three-dimensional volumetric measurements showed a much higher amount of swelling and shrinkage than those determined from one-dimensional measurements. Hydraulic hysteresis rapidly decreased with the swell-shrink cycles as a result of macro-structural stabilization. Under the no/low confining conditions, the swell-shrink cycles generally caused a decrease of global expansion and an increase of global shrinkage. Moreover, a special phenomenon showing a reduction of global shrinkage on significant drying was discovered in the less plastic soil sample. [ABSTRACT FROM AUTHOR]

We present a new, very simple to use and very easy to align, inexpensive, robust, mono-static optical hygrometer based on tunable diode laser absorption spectroscopy (TDLAS) that makes use of very inexpensive reflective foils as scattering targets at the distant side of the absorption path. Various alternative foils as scattering targets were examined concerning their reflective behaviour and their suitability for TDLAS applications. Using a micro prismatic reflection tape as the optimum scattering target we determined absolute water vapour concentrations employing open path TDLAS. With the reflection tape being in a distance of 75 cm to 1 m (i.e., absorption path lengths between 1.5 and 2 m) we detected ambient HO concentrations of up to 12,300 ppmv with detectivities of 1 ppm which corresponds to length and bandwidth normalized HO detection limits of up to 0.9 ppmv m/ $$ \sqrt {\text{Hz}} $$, which is only a factor of 2 worse than our previous bi-static TDLAS setups (Hunsmann, Appl. Phys. B 92:393-401, ). This small sensitivity disadvantage is well compensated for by the simplicity of the spectrometer setup and particularly by its extreme tolerance towards misalignment of the scattering target. [ABSTRACT FROM AUTHOR]

A novel, fiber-optic in situ laser hygrometer was developed to measure water vapor with microsecond time resolution directly inside an internal combustion (IC) engine. The instrument is intended for sampling-free quantification of recirculated exhaust gas in combustion engines. Direct tunable diode laser absorption spectroscopy was employed to allow absolute and self-calibrating HO measurements. The compact and user-friendly instrument combines a fiber-coupled, 1.37 μm distributed feedback diode laser with kHz-fast, continuous wavelength scanning. Only small, typically 10 mm, optical access ports in the engine are needed. The new in situ hygrometer was tested via measurements in a motored optical research engine operated on ambient air, without any artificial humidification. Scanning the laser at 4 kHz resulted in a time resolution of 250 μs (i.e., 3° crank angle at 2,000 rpm), while the DC-coupled detector signals are digitized with a 4MSamples/s 16-bit data acquisition system. Absolute water vapor concentrations around 1 vol.% could be measured and quantified during the full compression stroke, i.e., over a pressure/temperature range of 0.07-0.52 MPa/300-500 K. Without any scan averaging or bandwidth filtering we could demonstrate signal-to-noise ratios between 51 (at p = 0.1 MPa) and 33 (at p = 0.4 MPa), which corresponds to HO detection limits between 0.02 and 0.035 vol.% or length and bandwidth normalized detectivities of 285 and 477 ppb m Hz, respectively. Comparison of the dynamic HO behavior during the compression stroke across several engine cycles and different operating conditions showed good reproducibility and absolute accuracy of the results, consistent with the boundary conditions, i.e., motored air operation. This new sensor therefore opens up new possibilities for engine cycle-resolved, calibration-free in situ AGR quantification and optimization in engine applications. [ABSTRACT FROM AUTHOR]

In the field of humidity quantities, the first CIPM key comparison, CCT-K6 is at its end. The corresponding European regional key comparison, EUROMET.T-K6, was completed in early 2008, about 4 years after the starting initial measurements in the project. In total, 24 NMIs from different countries took part in the comparison. This number includes 22 EURAMET countries, and Russia and South Africa. The comparison covered the dew-point temperature range from −50 °C to +20 °C. It was carried out in three parallel loops, each with two chilled mirror hygrometers as transfer standards in each loop. The comparison scheme was designed to ensure high quality results with evenly spread workload for the participants. It is shown that the standard uncertainty due to the long-term instability was smaller than 0.008 °C in all loops. The standard uncertainties due to links between the loops were found to be smaller than 0.025 °C at −50 °C and 0.010 °C elsewhere. Conclusions on the equivalence of the dew-point temperature standards are drawn on the basis of calculated bilateral degrees of equivalence and deviations from the EURAMET comparison reference values (ERV). Taking into account 16 different primary dew-point realizations and 8 secondary realizations, the results demonstrate the equivalence of a large number of laboratories at an uncertainty level that is better than achieved in other multilateral comparisons so far in the humidity field. [ABSTRACT FROM AUTHOR]

A new humidity generator that produces gas streams of known moisture content at temperatures from 85 °C to 200 °C, absolute pressures from 0.2 MPa to 1.6 MPa, and relative humidities from 10 % to 90 % has been developed. The generator produces a moist gas stream by injecting fixed-rate streams of dry gas and liquid water into a vaporizer, where the water evaporates into the gas. The gas stream passes into a re-entrant radio-frequency (RF) cavity, which serves as our reference hygrometer, and then a test chamber. The present standard uncertainty of the RF hygrometer is 0.6 %, limited by the uncertainty of literature values for the polarizability of water. Dry nitrogen gas purging the pressure transducer line also combines with the moist gas stream downstream of the test chamber and flows through one of a set of capillaries. Modulation of gas flow through the fixed flow impedance of the capillary gives a simple method for controlling pressure. Individual insulated, temperature-controlled aluminum ovens enclose each major component. A larger oven encloses these ovens and their connecting tubing. To minimize corrosion, critical components are constructed of high-nickel alloys. The small total volume (<1 L) and small flow rate (<0.5 L·min) reduce operational hazards from steam scalding or from gas explosion. [ABSTRACT FROM AUTHOR]

Psychrometers remain the most widely used instruments for controlling the humidity in climatic test chambers, yet the calibration of these instruments is particularly challenging. Psychrometer calibrations require careful consideration of influence variables such as the fitting and cleanliness of the wick, the effect of the calibration chamber on the air flow past the sensors, on radiation incident on the sensors, and on the dissipation heat from the built-in fan (if included). In addition, uncertainty requirements for calibration of such psychrometers are typically around 1 %rh to 2 %rh, i.e., close to the best calibration and measurement uncertainties (CMCs) claimed by national metrology institutes (NMIs). As well as their role in supporting CMCs, inter-comparisons provide a good test-ground to ensure all influence variables are controlled or otherwise accounted for in the uncertainty budget. This paper presents the results of a comparison of psychrometer calibrations performed by the NMIs in Denmark, Slovenia, and Finland. The comparison was carried out under EURAMET Project No. 1033 with the aim to investigate the equivalence of psychrometer calibrations performed at the highest level and to gather practical experience to be used in similar comparisons in the future. An aspirated electro-psychrometer was used for the comparison, and calibrations were carried out in the range from 15 %rh to 93 %rh in a temperature range from 15 °C to 70 °C. While the results show good agreement at high relative humidity, significant differences at low relative humidity are reported. It is suggested that the differences are caused by a combination of psychrometer wick contamination and a difference in the wick-wetting methods used by the participant laboratories. [ABSTRACT FROM AUTHOR]

When a surface acoustic wave (SAW) is excited on a spherical surface, a naturally collimated SAW propagates around the equator hundreds of times. The propagation characteristics such as the velocity and amplitude are affected by adsorbed and/or reacted molecules on the surface, and the changes are accumulated by multiple turns of propagation. This enables highly sensitive detection of adsorbed molecules including water vapor. In this paper, the development and testing of a 1 mm diameter spherical SAW sensor, which is capable of measuring water vapor at concentrations well below 1 μmol · molHO in N, are reported. The rise time from 10% to 90% of the spherical SAW sensor to a step change from dry N to 1 μmol · molHO in N was approximately 15 s. [ABSTRACT FROM AUTHOR]

Precision optical dew-point hygrometers are the most commonly used transfer standards for the comparison of dew-point temperature realizations at National Metrology Institutes (NMIs) and for disseminating traceability to calibration laboratories. These instruments have been shown to be highly reproducible when properly used. In order to obtain the best performance, the resistance of the platinum resistance thermometer (PRT) embedded in the mirror is usually measured with an external, traceable resistance bridge or digital multimeter. The relation between the conventional calibration of miniature PRTs, prior to their assembly in the mirrors of state-of-the-art optical dew-point hygrometers and their subsequent calibration as dew-point temperature measurement devices, has been investigated. Standard humidity generators of three NMIs were used to calibrate hygrometers of different designs, covering the dew-point temperature range from −75 °C to + 95 °C. The results span more than a decade, during which time successive improvements and modifications were implemented by the manufacturer. The findings are presented and discussed in the context of enabling the optimum use of these transfer standards and as a basis for determining contributions to the uncertainty in their calibration. [ABSTRACT FROM AUTHOR]

The comparison of two high-range standard humidity generators used by Instituto Nacional de Técnica Aeroespacial to realize dew-point temperature in the range from −10 °C to +95 °C has been performed using state-of-the art transfer standards and measurement procedures, over their overlapping range from −10 °C to +75 °C. The aim of this study is to investigate the level of agreement between the two generators, to determine any bias, and to quantify the level of consistency of the two realizations. The measurement procedures adopted to minimize the effect of the influence factors due to the transfer standards are described, and the results are discussed in the context of the declared calibration and measurement capabilities (CMCs). [ABSTRACT FROM AUTHOR]

The continuous quest for improved specifications of optical dew-point hygrometers has raised customer expectations on the performance of these devices. In the absence of a long calibration history, users with a limited prior experience in the measurement of humidity, place reliance on manufacturer specifications to estimate long-term stability. While this might be reasonable in the case of measurement of electrical quantities, in humidity it can lead to optimistic estimations of uncertainty. This article reports a study of the long-term stability of some hygrometers and the analysis of their performance as monitored through regular calibration. The results of the investigations provide some typical, realistic uncertainties associated with the long-term stability of instruments used in calibration and testing laboratories. Together, these uncertainties can help in establishing initial contributions in uncertainty budgets, as well as in setting the minimum calibration requirements, based on the evaluation of dominant influence quantities. [ABSTRACT FROM AUTHOR]

During calibration of precision optical dew-point hygrometers (DPHs), it is usually necessary to take into account the pressure drop induced by the gas flow between the 'point of reference' and the 'point of use' (mirror or measuring head of the DPH) either as a correction of the reference dew-point temperature or as part of the uncertainty estimation. At dew-point temperatures in the range of ambient temperature and below, it is sufficient to determine the pressure drop for the required gas flow, and to keep the volumetric flow constant during the measurements. In this case, it is feasible to keep the dry-gas flow into the dew-point generator constant or to measure the flow downstream the DPH at ambient temperature. In normal operation, at least one DPH in addition to the monitoring DPH are used, and this operation has to be applied to each instrument. The situation is different at high dew-point temperatures up to 95 °C, the currently achievable upper limit reported in this paper. With increasing dew-point temperatures, the reference gas contains increasing amounts of water vapour and a constant dry-gas flow will lead to a significant enhanced volume flow at the conditions at the point of use, and therefore, to a significantly varying pressure drop depending on the applied dew-point temperature. At dew-point temperatures above ambient temperature, it is also necessary to heat the reference gas and the mirror head of the DPH sufficiently to avoid condensation which will additionally increase the volume flow and the pressure drop. In this paper, a method is provided to calculate the dry-gas flow rate needed to maintain a known wet-gas flow rate through a chilled mirror for a range of temperature and pressures. [ABSTRACT FROM AUTHOR]

The KRISS low frost-point humidity generator (LFPG), was developed in 2006 for the frost-point range (−95 to −40)° C in order to extend calibration capabilities. In this paper, the evaluation of the generator's uncertainty budget is reported for which each uncertainty component was categorized and estimated by experiment and calculation. The uncertainty of the LFPG depends on the generated frost point, gas flow rate, and change of moisture concentration in transportation. The standard uncertainty of LFPG is less than 32 mK in the frost-point range from −70 ° C to −40 ° C. However, in the lower frost-point range, the uncertainty increases to 137 mK at −90 ° C, and this is mainly due to water adsorption or desorption in the transportation tubing from saturator to hygrometer. [ABSTRACT FROM AUTHOR]

The water-saturated phase relations have been determined for a primitive magnesian andesite (57 wt% SiO, 9 wt% MgO) from the Mt. Shasta, CA region over the pressure range 200-800 MPa, temperature range of 915-1,070 °C, and oxygen fugacities varying from the nickel-nickel oxide (NNO) buffer to three log units above NNO (NNO+3). The phase diagram of a primitive basaltic andesite (52 wt% SiO, 10.5 wt% MgO) also from the Mt. Shasta region (Grove et al. in Contrib Miner Petrol 145:515-533; ) has been supplemented with additional experimental data at 500 MPa. Hydrous phase relations for these compositions allow a comparison of the dramatic effects of dissolved HO on the crystallization sequence. Liquidus mineral phase stability and appearance temperatures vary sensitively in response to variation in pressure and HO content, and this information is used to calibrate magmatic barometers-hygrometers for primitive arc magmas. HO-saturated experiments on both compositions reveal the strong dependence of amphibole stability on the partial pressure of HO. A narrow stability field is identified where olivine and amphibole are coexisting phases in the primitive andesite composition above 500 MPa and at least until 800 MPa, between 975-1,025 °C. With increasing HO pressure ( $${P}_{\text {H}_2{\rm O}}$$), the temperature difference between the liquidus and amphibole appearance decreases, causing a change in chemical composition of the first amphibole to crystallize. An empirical calibration is proposed for an amphibole first appearance barometer-hygrometer that uses Mg# of the amphibole and $$f_{\text {O}_2}$$:This barometer gives a minimum $${P}_{\text{H}_{2}{\rm O}}$$ recorded by the first appearance of amphibole in primitive arc basaltic andesite and andesite. We apply this barometer to amphibole antecrysts erupted in mixed andesite and dacite lavas from the Mt. Shasta, CA stratocone. Both high HO pressures (500-900 MPa) and high pre-eruptive magmatic HO contents (10-14 wt% HO) are indicated for the primitive end members of magma mixing that are preserved in the Shasta lavas. We also use these new experimental data to explore and evaluate the empirical hornblende barometer of Larocque and Canil (). [ABSTRACT FROM AUTHOR]

A system for diagnosing emergency states from the moisture content of high-power water-cooled turbogenerators is presented. The use of the differential method of measurement and a consideration of the particular features of the generators has enabled high accuracy and reliability of the results of the measurements to be obtained. Experimental samples of the system are being successfully used in the Ryazan State Regional Power Plant (GRÉS). [ABSTRACT FROM AUTHOR]

We describe a new differential analyzer implementing a technique for studying the hygroscopic properties of filter-precipitated aerosol particles based on a katharometric measurement of the amount of water vapor absorbed by an aerosol sample. The instrument has been tested on particles of sodium chloride and ammonium sulphate. A comparison of our data with the results of analogous measurements described in literature shows that the suggested technique has several important advantages, i.e., (1) higher sensitivity, which allows the measurement of particles of no more than 0.1 mg in mass; (2) a wide range of relative humidity variations (up to 99% at an accuracy of 0.06% RH at the upper-range value); (3) the absence of intermediate measurement steps connected with the need to account for water absorption by elements of a measurement system; and (4) the absence of restrictions to the size and shape of particles studied. In addition, the measurement technique rules out the possibility of errors caused by the kinetic effects of mass transport in the aerosol-water vapor system. [ABSTRACT FROM AUTHOR]

We present data from studies of coulometric electrolytic cells with platinum, rhodium, and platinum-iridium electrodes. We describe the experimental apparatus, the procedure, and the results of a study of the dependence of the measurement error in analysis of inert gases, nitrogen, hydrogen, and oxygen at temperatures of 20 and 50°C. We give recommendations for use of coulometric electrolytic cells in hygrometers with measurement range 0-1000 ppm. [ABSTRACT FROM AUTHOR]

One of the most important components of the error of flow-through coulomometric hygrometers is that due to “moisture transit,” or loss. Exact relationships between this error and the fraction of electrolytic current from the control part of a coulometric cell for both fresh (“new”) cells (i.e., those that been regenerated) and cells that are in the aging process are obtained. It is shown that it is possible to determine and take into account this error in the measurement process. [ABSTRACT FROM AUTHOR]

We probed four closely spaced rovibrational water vapor absorption transitions near $\tilde{\nu}$ =7100 cm-1 using frequency-stabilized cavity ring-down spectroscopy. Room-temperature spectra were acquired for pure water vapor in the Doppler limit and for mixtures containing ≈6.6 μmol mol-1 of water vapor in N2 at total gas pressures ranging from 6.5 kPa to 53 kPa. By measuring Lamb dips for each transition, we demonstrated a resolution of 50 kHz and determined relative transition frequencies with an uncertainty <0.4 MHz over a 10 GHz range. Pressure-induced broadening, collisional narrowing coefficients of the component transitions and line areas were retrieved by fits of model line shapes to the measured spectra. Standard and advanced models were considered including those which incorporated the combined effects of collisional narrowing and speed-dependent line broadening and line shifting. By measuring water vapor concentration with a transfer standard hygrometer, we determined line intensities in terms of measured line areas with a combined relative uncertainty of 0.6%. [ABSTRACT FROM AUTHOR]

The eddy-covariance method is the primary way of measuring turbulent fluxes directly. Many investigators have found that these flux measurements often do not satisfy a fundamental criterion—closure of the surface energy balance. This study investigates to what extent the eddy-covariance measurement technology can be made responsible for this deficiency, in particular the effects of instrumentation or of the post-field data processing. Therefore, current eddy-covariance sensors and several post-field data processing methods were compared. The differences in methodology resulted in deviations of 10% for the sensible heat flux and of 15% for the latent heat flux for an averaging time of 30 min. These disparities were mostly due to different sensor separation corrections and a linear detrending of the data. The impact of different instrumentation on the resulting heat flux estimates was significantly higher. Large deviations from the reference system of up to 50% were found for some sensor combinations. However, very good measurement quality was found for a CSAT3 sonic together with a KH20 krypton hygrometer and also for a UW sonic together with a KH20. If these systems are well calibrated and maintained, an accuracy of better than 5% can be achieved for 30-min values of sensible and latent heat flux measurements. The results from the sonic anemometers Gill Solent-HS, ATI-K, Metek USA-1, and R.M. Young 81000 showed more or less larger deviations from the reference system. The LI-COR LI-7500 open-path H2O/CO2 gas analyser in the test was one of the first serial numbers of this sensor type and had technical problems regarding direct solar radiation sensitivity and signal delay. These problems are known by the manufacturer and improvements of the sensor have since been made. [ABSTRACT FROM AUTHOR]

A fluorescent airborne stratospheric hygrometer (FLASH) for precise water vapor measurements on board a high-altitude airplane in the upper troposphere and the lower stratosphere has been developed and produced at the Central Aerological Observatory. The range of measured water vapor concentrations is 0.5–300 ppmv at altitudes of 8–21 km. The results of laboratory tests of the instrument are compared to the data of the other hygrometers. Some results of water vapor measurements on board the Geofizika M55 high-altitude airplane in different regions of the Earth are presented. [ABSTRACT FROM AUTHOR]

Recent studies have shed new light on humidity conditions in the polar atmospheric boundary layer, and cast doubt on the reliability of humidity measurements above the frost point humidity. These issues are addressed herein by considering the processes that affect humidity and its measurement in cold climate conditions and by analyzing observations from two sites, at which the relative humidity is frequently above 100% at sub-freezing temperatures, as shown by repeatedly observed in-cloud icing events. Humidity measurements were made at these two sites by the commonly used Vaisala HMP35 probe and HMP233 capacitance probes respectively and simultaneously by the same manufacturer’s HMP243 probe that determines the frost point by a heated capacitance sensor. The results confirm that the relative humidity is frequently well above the frost point, both due to radiative cooling and to the advection of moist air, and that the conventional humidity measurements are unable to detect these events. Furthermore, after such events, the iced sensors show too high a humidity. The false values occur due to ice growth on the probe and cannot be corrected by any algorithm. Our results indicate that these problems, inherent to conventional humidity measurements in cold and humid environments, are avoided by the use of HMP243, which has a heated humidity probe. [ABSTRACT FROM AUTHOR]

The inability to measure humidity during stimulation has so far prevented us from understanding the contribution of moist cells and dry cells to orientation in a gradient of humidity. The problem was solved in the present study by means of a UV-absorption hygrometer that made it possible to monitor humidity at a rate of 100 Hz. The antennal moist and dry cells of the cockroach were exposed to humidities alternatively falling or rising at low rates between -1% RH s-1 and +1% RH s-1 (relative humidity). Impulse frequency of both types of cells depended simultaneously on instantaneous humidity and its rate of change. High frequencies of the moist cells signal high humidity. But at a given humidity, the response frequency is higher still when humidity is also rising. Conversely, high frequencies of the dry cell signal low humidity, and frequency is higher still at a given humidity when humidity is also falling. These responses ensure that the cockroach spent a minimum time in environments where desiccation or hydration occur and may thus protect the animal from emerging accidentally from under cover into moving air. In the constant-humidity retreat of the cockroach, fluctuating or even drifting discharge frequencies could serve as an early warning: return! [ABSTRACT FROM AUTHOR]

In the cold Canadian environment, humidity measurements can be very difficult to conduct. In this brief communication, humidity observations taken by two different sensors at six remote Canadian Arctic locations are compared. The observations collected by Vaisala HMP35CF sensors display a strong tendency toward the ice saturation point whereas dew cell instruments exhibit significantly lower values of relative humidity with respect to ice (RHi). Humidity data collected by HMP35CF hygrometers are therefore unreliable since they are subject to persistent icing that lead them to record values of RHi near 100%, irrespective of the air temperature. The high humidity bias emerges at meteorological stations mounted with the HMP35CF probes since these instruments are usually neither sheltered nor heated, and are not attended to at regular intervals. Thus, great care must be taken in utilizing humidity data recorded by HMP35CF sensors across the network of climate autostations in Canada. [ABSTRACT FROM AUTHOR]

Abstract. We describe a near-IR diode laser hygrometer developed for in-situ measurement of water vapor from airborne platforms throughout the troposphere. The sensor is based on the absorption of a 1.39-micro m diode laser beam and employs an open, folded 50-cm absorption path placed in the free-stream air using an external probe. Water vapor number density is continuously recorded at 10Hz. Extensive laboratory testing showed a sensitivity of 10[sup 12] molecules cm[sup -3] Hz[sup -1/2] (signal-to-noise ratio 3). This is equivalent to a mixing ratio of 0.3 ppmv at average midlatitude tropopause conditions or a mixing ratio of 0.6 ppmv under boundary-layer conditions. The corresponding minimum measurable absorbance is 10[sup -5] Hz[sup -1/2]. The laser hygrometer was field-demonstrated aboard the NASA P3B research aircraft, during a series of flights spanning several weeks in the summer of 1997. During this demonstration, the laser hygrometer was intercompared with two optical chilled mirror hygrometers. In general, the laser hygrometer performed well; however, under some conditions, it reported water vapor number densities 20% greater than the cryogenic frost-point hygrometer. This difference is currently under study. [ABSTRACT FROM AUTHOR]

In this work, a Raman spectroscopic study of nanoporous silicon sensor samples demonstrated its use as a method of gauging the sensor potential via quantitative data it provides on the sensor nanostructure dimensions. This special property of the Raman spectroscopy technique also showed its potential to determine mechanical stability of the samples over 3 months. This work also shows that the Raman spectroscopy technique is sensitive to step changes in relative humidity in all the sensor samples via its measurement of the strain-free crystalline silicon (c-Si) Raman peak. Since the Raman technique is non-destructive and senses remotely on the fragile nanoporous sensor samples it will be the ideal replacement of the presently used electrical capacitance techniques as the primary determination of relative humidity. [ABSTRACT FROM AUTHOR]

MBW Calibration AG (MBW) is the Designated Institute (DI) for humidity appointed by the Federal Institute of Metrology, METAS. MBW currently offers calibration and measurement capabilities (CMC) for frost/dew-point hygrometers by comparison with precision chilled-mirror transfer standards that have been calibrated using the primary standards of leading European National Metrology Institutes or DI. The design, construction and validation of two standard humidity generators to be used as the Swiss national standards for the primary realization of frost/dew-point temperature in the range from − 90 °C to + 95 °C are presented and discussed. The generators are operated as continuous flow “single-pressure” generators in the range from − 80 °C to − 10 °C with saturation over ice and from 0.5 °C to + 95 °C with saturation over water. Additionally, they are used in “two-pressure” mode for saturation over ice down to frost-point temperatures of − 90 °C and down to − 20 °C for saturation over water. The main saturators of both generators have been designed to fit in commercially available calibration baths with either ethanol or distilled water as the heat transfer fluid for saturator temperatures below and above 0 °C, respectively. Saturator temperature is measured using standard platinum resistance thermometers and a purpose-built precision thermometer. Pressure measurements are taken with gauge pressure transducers and a separate barometric sensor, to reduce the influence of the atmospheric pressure on the measurement of the pressure ratio and make full use of the correlation of pressure measurements and enhancement factors when operating in two-pressure mode. A totally automated pre-saturation and flow control system facilitates the calibration of state-of-the-art chilled-mirror transfer for standards without manual readjustment of the generated flowrate to ensure a constant volumetric flow at the conditions of the mirror. The uncertainty budget leading to the CMC for frost/dew-point temperature realization is presented in the context of the experimental validation performed. The results in the overlapping range of both generators are presented and used as further evidence of the saturation efficiency of both standards. [ABSTRACT FROM AUTHOR]

This paper gives a detailed description of a new chamber designed for calibration of relative humidity (RH) instruments at Laboratory for Process Measurement (HMI/FSB-LPM). To the present time, the calibrations of RH instruments at the HMI/FSB-LPM were done by comparison method using a climatic chamber of large volume and calibrated dew point hygrometer with an additional thermometer. Since 2010, HMI/FSB-LPM in cooperation with Centre for Metrology and Accreditation in Finland (MIKES) developed the two primary dew point generators which cover the dew point temperature range between -70∘C and 60∘C. In order to utilize these facilities for calibrations of the RH instruments, the new chamber was designed, manufactured and installed in the existing system, aiming to extend its range and reduce the related calibration uncertainties. The chamber construction allows its use in a thermostatic bath of larger volume as well as in the climatic chambers. In the scope of this paper, performances of the new chamber were tested while it was submersed in a thermostated bath. The chamber can simultaneously accommodate up to three RH sensors. In order to keep the design of the chamber simple, only cylindrical RH sensors detachable from display units can be calibrated. Possible optimizations are also discussed, and improvements in the design proposed. By using the new chamber, HMI/FSB-LPM reduced the expanded calibration uncertainties (level of confidence 95 %, coverage factor k=2) from 0.6 %rh to 0.25 %rh at 30 %rh (23∘C), and from 0.8 %rh to 0.53 %rh at 70 %rh (23∘C). [ABSTRACT FROM AUTHOR]